(1) Field of the Invention
The present invention relates to electrical power converters and particularly to solid state circuits which may be utilized to supply a high DC voltage to an intermittently energized load, such as a gaseous discharge tube, from a low voltage direct current source. More specifically, this invention is directed to the furnishing of power to and the exercise of control over light generators, especially flash tubes, which are periodically energized to produce a preselected pattern of light emissions. Accordingly, the general objects of the present invention are to provide novel and approved apparatus and methods of such character.
(2) Description of the Prior Art
While not limited thereto in its utility, the present invention is well-suited for controlling the operation of warning lights and particularly for employment in warning light systems which include xenon flash tubes. Such warning light systems are well-known in the art and find application on emergency vehicles, aircraft and in other installations where it is considered necessary or desirable to attract attention by means of the generation of intermittent bursts of energy in the visible range of the frequency spectrum. For a disclosure of prior art power supplies for controlling the energization of gaseous discharge tubes, reference may be had to U.S. Pat. Nos. 3,515,973; 4,013,921 and 4,321,507.
Warning light systems are generally characterized by the type of light generator employed, i.e., an incandescent lamp or a gaseous discharge tube. With both types of light source, in order to enhance visibility, the system will cause light to be generated in pulses, i.e., a flashing light will attract attention much more readily than a steady light. Both types of light source have been found to have attributes and disadvantages. In order to enhance the visibility of the light produced by means of a gaseous discharge tube, power supply circuits have been devised which will cause such tubes to "fire" in a pattern of two to four intense flashes spaced closely in time followed by an "off" time, during which the energy storage capacitance of the power supply is recharged, the "off" time comprising 80% or more of the cycle. In the past, the off time between the individual flashes of such a serial pattern was, at minimum, 125 milliseconds while the duration of the flash was approximately 1 millisecond. Thus, notwithstanding the retention properties of the human eye, each individual flash was discernable and, most importantly, the off time comprised the major part of the cycle.
It should be apparent from the above discussion that there has been a long-standing desire to decrease the time between successive energizations of a gaseous discharge tube, whereby a series of pulses would be perceived by an observer as a single long duration flash, and to simultaneously increase the number of pulses in a series thus increasing the perceived on-time of the flash tube. However, in seeking to extend the perceived flash duration, restraints have been placed upon the power supply designer. Firstly, the overall physical size of the power supply and its power consumption had to remain reasonable and, in fact, was determined by the expected usage in vehicle applications. Secondly, the cost of the power supply could not place the flash tube type warning light system at a competitive disadvantage vis-a-vis a system employing incandescent lamps. Additionally, product reliability could not be compromised by, for example, subjecting components to excessive current flow or temperature.